Science —

Platypus genome as distinctive as its owner

Researchers have completed the genome sequence of the platypus, an animal with …

The platypus has such a bizarre mix of features that the first samples sent back to Europe were examined for signs of fraud—biologists suspected a trickster had stitched together pieces of different animals. It and its lone fellow Monotreme, the echidna, have the fur and temperature regulation of a mammal, but lay eggs through a single reproductive/excretory orifice like reptiles. Adding to the confusion, the well-defined fossil series that details the evolution of mammals from therapsid reptiles trails off around the time that monotremes split off from the rest of mammals about 170 million years ago. Now, a few of the mysteries have been cleared up, thanks to the completion of the platypus genome, published today in Nature.

The paper starts off with a nice description of the agglomeration of features distinct to the monotremes. Although they lay eggs, the young that hatch are not fully developed, being more akin to those of marsupials at birth. The young require several months of nursing; platypuses produce milk, but provide it to their young without the benefit of nipples. Male platypuses have testes that remain internal; they also produce venom, but inject it via a hindlimb claw. Platypuses are also unique among mammals in that they have an electrosensory system.

That weird collection of features has been known to extend to the genome level, and the completed sequence greatly expands on this. The platypus has a whopping 52 chromosomes, many of them quite small, a configuration that looks like those of birds and reptiles. Its sex chromosomes are inherited like the mammalian X/Y system, but share sequence homology to the birds' Z/W chromosomes and, in a uniquely platypusian twist on things, the animals run around with ten sex chromosomes (males have five each of the X and Y). Incidentally, birds, being derived from dinosaurs, are used as a reptilian reference genome in the absence of having sequenced anything that's commonly thought of as a modern reptile.

Platypuses have an extensive collection of non-coding RNAs. Many of their microRNAs are shared with birds and mammals, and only small fractions are shared with only one or the other of these groups. But roughly half of the microRNAs found were specific to monotremes, and many of these appear to be involved in reproduction in some way. All eukaryotes also make a class of non-coding RNA called snoRNA that helps in the processing of ribosomal RNAs. In the platypus, however, snoRNAs have somehow latched on to a reverse-transcription based transposase, and have hopped all over the genome, resulting in a gross overabundance of these sequences. Actual transposons account for roughly half the genome, and it appears that some of these may still be active.

At the gene level, the platypus appears to have about 18,500 protein coding regions, roughly in line with the count in other mammals, but a bit higher than the chicken; 82 percent of these appear in one of the other species of mammals or birds. The genes involved in fertilization are a mix of those found only in mammals and those found only in birds and fish. The platypus also has only a single gene for egg yolk protein, in contrast to the three in the chicken genome. Like the rest of the mammals, the genes for platypus milk proteins are clustered together and reside next to the tooth enamel genes that they arose from via duplication.

The genome also sheds light on some of the unique platypus features. Like other mammals, the platypus has expanded their sensory abilities by duplicating many of the genes involved in smell. Oddly, in the platypus, the family undergoing the largest duplication is exclusively used for pheromone sensing in the mice. The authors suggest that these were adapted for use by the electrosensing system of the animal. The platypus' venom appears to be derived by duplication of many of the same genes that have given rise to components of reptilian venom. Most of these genes have been duplicated anyway on the mammalian lineage, but a separate duplication event created a side branch of genes that were exapted into the venom system.

It's easy to think of this mix of mammalian and reptilian/avian characteristics in the genome as simply reflecting the biology of the platypus itself. But the genome goes a long way towards clarifying what systems are truly inherited from a common ancestor (sex chromosomes, for example), which date from the ancestors of all mammals (milk proteins), and which are uniquely montreme (venom genes). In that sense, it tells us a lot more about what's uniquely mammalian and, in that respect, it tells us much about ourselves.